Blender Cycles normal map difficulty
Terminator artifacts
As gandalf3 pointed out, you are suffering from terminator artifacts. I have seen this before in my own models, but didn't know that there is a name for it :-). This is one issue with normal mapping. Blender uses the normal map to calculate the final shading at a particular point on the model, but it can't use the normal map to determine whether or not the light is hitting part of the model. Blender only uses the physical model to calculate where the light hits. Normal maps are only a form of trickery used to make the model appear to be higher resolution than what they actually are; they don't really add new geometry.
The reason it didn't appear when you were using Blender internal is probably because the lighting was different (judging from the picture). You will have to either use subdivision to make the lighting calculations more accurate, or adjust your lighting so that the dark area is lit.